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Biomechanics -- Defined

- Bio - life; living organism
- Mechanics - the branch of physics concerned with the analysis of the action of forces on matter or material systems
- Biomechanics – the study of forces and their effects on living systems
- Exercise & Sport Biomechanics – the study of forces and their effects on humans in exercise and sport
- Applied or “Functional” Biomechanics – (the focus of this class); the examination of the application of biomechanics in the exercise and sports field

Human Biomechanics

- Applications of biomechanics (human biomechanics)
- Purpose of the science – understand, protect and enhance human function
- Role in sport – ultimately, to improve performance
- Role in therapy – rehabilitate, re-educate
- Role in product design – to design products that

optimally support human function

- Role in injury prevention – to minimize adverse stress and strain on the body through movement analysis, technique design and product development
- Role in the workplace – Ergonomics - to maximize productivity by minimizing worker fatigue and discomfort
- Who uses biomechanics?

Mechanics - analysis of the action of forces on matter or material systems

Mechanics

Deformable Body Mechanics

Fluid Mechanics

Rigid Body Mechanics

Relativistic Mechanics

Quantum Mechanics

Rigid Body – objects are assumed to be perfectly rigid

Deformable Body – objects can be deformed by a force

Fluid – Gas or fluid

Humans – Rigid or Deformable?

- Biological tissue, including the human body, is by nature, deformable. It can absorb forces, it can stretch, bend, compress.
- With regards to gross human movement, these deformations are relatively small, and for the sake of simplicity, Applied or Functional Biomechanics largely ignores these properties.
- Each segment of the body is considered a rigid body linked together by joints.
- In reality, repeated plastic deformation of biological tissue will result in injury.

Terminology Review

- Elasticity – property of a material demonstrated by its ability to return to its original length after the removal of a deforming force
- Plasticity – property of a material demonstrated by remaining permanently deformed after the removal of a force
- Viscoelasticity – property of a material expressed by a changing stress-strain relationship over time; the stress-strain properties of the material are rate dependent
- Stress – internal resistance generated as a tissue resists its deformation, divided by its cross sectional area
- Strain – the ratio of the tissue’s deformed length to its original length.
- Stiffness – the ratio of Stress : Strain
- Creep – a progressive strain of a material when exposed to a constant load over time i.e. intervertebral disc compression

Stress – Strain Curve

- Import curve here

Repetitive or prolonged stress at this strain % will eventually result in microdamage

(i.e. stress fracture)

Bone Stress-Strain Curve

Bone

Foam

Bone is relatively rigid – note the resistance to strain

Boney body segments determine human rigidity in biomechanical terms

Branches of Rigid Body Mechanics

Rigid Body

Mechanics

Statics

Dynamics

Statics – mechanics of objects

Kinematics

Kinetics

at rest, or at constant velocity

Dynamics – mechanics of objects in accelerated motion

Kinematics – describes the motion of a body without regard to the forces or torques that may produce the motion

Kinetics– describes the effect of forces on the body; i.e.. muscular force, gravitational force, external resistance force, ground reaction force, etc.

Basic Dimensions and Units of Measurement Used in Mechanics & Biomechanics

- Biomechanics is a quantifiable science, measurable, and can be expressed in numbers
- Systeme-Internationale d’Unites (SI Units)
- Length – measured in meters (m)
- Time – measured in seconds (s)
- Mass – measured in kilograms (kg), the measure of inertia, or resistance to a change in motion of an object

Mass vs. Weight

Massis the measure of inertia, whereas Weightis the measure of the force of gravity acting on an object.

Additional Dimensions & Units of Measure

- Length – millimeter (mm), centimeter (cm), kilometer (km), etc. are all based on the meter (m)
- Time – Minutes, hours, days, weeks, months, years, etc. can all be derived from the second (s)
- Mass – milligram (mg), gram (g), etc. are all based on the kilogram (kg)

Forces & Torques

- Force – a push or pull; exerted by one object on another; come in pairs (Newton’s 3rd Law); creates acceleration or deformation (Newton’s 2nd Law); causes an object to start, stop, change direction, speed up or slow down (Newton’s 1st Law)
- SI Unit of Force is the Newton (N) = force required to accelerate a 1 kg mass 1/m/s/s
- Force is described by its size (magnitude) and direction
- The angular equivalent of F is Torque (T); a Torque rotates an object about an A of R
- T = F x moment arm
- Resultant Force – the summation of all forces acting on a body; determines the direction of the body

Forces (cont.)

- Internal Forces and Torques– forces or torques that act within the studied object; i.e. the human body, or the object being manipulated by the human; pole vault, soccer ball, etc. Internal forces can cause movement of body segments at a joint but cannot produce a change in the motion of a body’s C of M. Muscular force is the primary internal force examined in biomechanics. As the overwhelming majority of motion in the human body is angular, torque forces are more applicable in biomechanics.

(The terms Force and Torque will be used interchangeably throughout this course. Essentially, if the term “Force” is used to describe angular motion, "Torque” is implied.)

Forces (cont.)

- External Forces– forces that act on an object as a result of its interaction with the environment surrounding it
- Most External Forces are contact forces, requiring interaction w/ another object, body or fluid
- Some External Forces are non-contact forces; including gravitational, magnetic and electrical forces
- The science of biomechanics largely deals with contact forces and gravity (weight), which accelerates objects at 9.8 m/s
- Contact forces can be sub-divided into normal reaction force and friction

Contact Forces

Normal Reaction Force –

line of action of the force is

perpendicular to the surfaces in

contact

Friction Force –line of action

of the force is parallel to the

surfaces in contact

Newton’s Laws of Motion

- Newton’s Laws help to explain the relationship between forces and their impact on individual joints, as well as on total body motion.
- Knowledge of these concepts can help one understand athletic movement, improve athletic function, understand mechanisms of injury, treat and prevent injury

Newton’s Laws (cont.)

- Newton’s 1st Law – Law of Inertia
- A body remains at rest or in motion except when compelled by an external force to change its state. A force is required to start, stop, or alter motion
- Inertia – the tendency of a body to remain at rest or resist a change in velocity
- Inertia is directly proportional to its mass
- The angular equivalent is Mass Moment of Inertia

Mass Moment of Inertia

- Mass Moment of Inertia (I)– The resistance to change in a body’s angular velocity
- Dependent on both the objects mass and on the distribution of mass about it’s axis of rotation
- Radius of Gyration (p) –the average distance between the A of R and the C of M of a body
- With an increase in an object’s mass, or with an increase in the radius of gyration, I increases

I = mass of the object multiplied by the square of the R of G

I = m x p2

Law of Inertia – Biomechanical Application

- How can an athlete control their Mass Moment of Inertia? In other words how can they manipulate the resistance to change in angular velocity to attain a goal?

Newton’s Laws (cont.)

- Newton’s 2nd Law – Law of Acceleration
- The acceleration of a body is directly proportional to the F causing it, takes place in the same direction in which the F acts, and is inversely proportional to the mass of the body
- A = velocity / time (velocity = displacement / t)
- F = ma (Force = mass x acceleration) (linear)
- Angular equivalent of F is Torque (T)
- T = F x moment arm (rotational force applied to the A of R, through a moment arm)
- T has the same relationship with direction and mass moment of inertia as F has with direction and mass (inertia)
- As I (moment of Inertia) increases (due to increased R of G or increased mass), Acceleration decreases

Newton’s Laws (cont.)

- Newton’s 2nd (cont.)
- Impulse-Momentum Relationship – from F=ma, we can derive Momentum (p) and Impulse
- Impulse = Force x time (Ft)
- Momentum = mass x velocity (mv)
- Ft = mv (impulse = momentum)
- If Ft increases, mv increases
- Mass is considered constant

within biomechanics, therefore,

an increase in impulse implies an

increase in velocity

- How are the principles of

Impulse and Momentum

used in the design of sports

equipment?

Newton’s 2nd (cont.) Impulse-Momentum

- Because Mass is constant, and because external forces are largely non-modifiable, in the world of sports and exercise, the duration of force application is the most modifiable
- If the Force is not constant, impulse is the avg. force times the duration of that average force
- Essentially, calculating force as average force holds that force as a constant, however it is the peak force that we need to minimize
- If the application of Force is prolonged (increased time), in order to maintain the same magnitude of impulse (Ft), the Force magnitude (average and peak) must be lowered
- Conversely, if the application

of Force happens more rapidly

(decreased time), there will be a

higher Force (avg. & peak) in

order to maintain impulse

Newton’s Laws (cont.)

- Newton’s 2nd Law (cont.)
- Work-Energy Relationship -- from F=ma,

we can also derive Work (W)

- Work = Force x Distance

(W = FD) (linear)

- Angular equivalent = Torque x Angular displacement (T x degrees)
- Measured in Newton meters (Nm)

Work is a measure of strength,

measured by the extent to

which a force moves a body over

a distance without regard to time

Newton’s Laws (cont.)

- Newton’s 2nd (cont.)
- Power (P) – the rate of work; W/time
- W/t = F x D/t or F x Velocity (W=FV)
- Training power in an athlete requires doing work quickly, or explosively
- How is Power measured and trained in sport and exercise?

Newton’s Laws (cont.)

- Newton’s 3rd Law – Law of Action-Reaction
- For every action, there is an equal and opposite reaction
- The two bodies react

simultaneously, according

to F=ma ; each body

experiences a different

acceleration effect which

is dependent on its mass

References

- Neumann, D.A. (2002). Kinesiology of the Musculoskeletal System. St. Louis, Missouri. Mosby.
- McGinnis, P.M. (2005). Biomechanics of Sport and Exercise 2nd ed. Champaign, IL. Human Kinetics.

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